Optical gas sensors have been proposed that make use of the swelling of polymers by measuring the change in optical thickness of the polymer. Several detection techniques have been used such as surface plasmon resonance (Matsui et al. Anal. Chem. 77 4282 (2005)), interference (Schalkhammer et al. Sensors and Actuators B 24 166 (1995)) and fluorescence (McCurley Biosensors & Bioelectronics 9 527 (1994)).
Matsui et al. (Anal. Chem. 77 4282 (2005)) prepared a molecularly imprinted polymer gel with embedded gold nanoparticles on a gold substrate of a chip to obtain surface plasmon resonance (SPR). The sensing is based on swelling of the imprinted polymer gel that is triggered by an analyte binding event within the polymer gel. The swelling causes a greater distance between the gold nanoparticles and the substrate, shifting a dip of the SPR curve to a higher SPR angle. Although the SPR technique can be very sensitive, a disadvantage is that it requires the coupling of polarized monochromatic light to a metal surface using prisms, grating couplers, or integrated optical waveguides structures.
Schalkhammer et al. (Sensors and Actuators B 24-25 166 (1995)) used an optical thin-film resonance system consisting of a metal mirror, covered by a thin layer of an optically transparent pH-sensitive swelling polymer and a metal-island film. The system is characterized by a narrow reflection minimum, whose spectral position shifts with the interlayer thickness. By monitoring the slope of the characteristic (narrow-bandwidth) reflection minimum, 1% change of the absorption could be measured at a fixed wavelength for a change of 0.1% of the polymer thickness. A disadvantage of this method is that the interlayer thickness is dictated by the optical resonance condition of the metal-island/polymer/mirror system, leading to an interlayer thickness of several hundreds of nanometers for reflection minimum in the visible range. The sensor's response time can therefore not be reduced by decreasing the thickness of the interlayer.
McCurley (Biosensors and Bioelectronics 9 527 (1994)) use(s) fluorophores in a hydrogel. Protonation of the amine moiety in the hydrogel caused the gel to expand. An increase in the gel volume resulted in a decrease in the concentration of the dye in the gel since the amount of the dye in the gel remains constant, leading to decrease in fluorescence intensity. A slow response time ˜10 min was observed, probably due to the need for the solvent to diffuse into the gel matrix.
Nevertheless, a need remains for gas sensing devices of low complexity, small size, and high sensitivity.